The present invention relates generally to magnetic resonance spectroscopy (MRS) and, more particularly, to a method and apparatus for single volume element (voxel) MRS that provides metabolite concentration values for identified tissues within a single voxel.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR spectroscopy is a common MR technique used for the determination of individual chemical compounds or metabolites located within a volume-of-interest (VOI). Single voxel MRS determines those metabolites on a per-voxel basis. The underlying principle of MRS is that atomic nuclei are surrounded by a cloud of electrons which slightly shield the nucleus from any external magnetic field. As the structure of the electron cloud is specific to an individual molecule or compound, the magnitude of this screening effect is then also a characteristic of the chemical environment of individual nuclei. Since the resonance frequency of the nuclei is proportional to the magnetic field it experiences, the resonance frequency can be determined not only by the external applied field, but also by the small field shift generated by the electron cloud. Detection of this chemical shift is usually expressed as “parts per million” (ppm) of the main frequency.
A drawback of conventional MRS techniques is the inability to segment or compartmentalize the tissues within a single voxel and, thus, provide a per-tissue type per-voxel metabolite concentration. Usually, a voxel is defined as cubic region of interest. As tissue types rarely have rectangular or cubic shapes, it is common for a single voxel to contain multiple tissue types. Different tissue types, however, have different metabolite concentrations contributing to the final spectrum. For example, for brain imaging, it is not uncommon for a given voxel to contain white matter, gray matter, and cerebral spinal fluid (CSF). Since conventional single voxel MRS generates a spectrum from all the metabolites in the voxel, it is not feasibly possible to ascertain what tissue is contributing to the overall spectrum for a given voxel. That is, the resulting spectrum is defined as the sum of all signals from all contributing tissue types in the voxel. Thus, it is possible for the metabolite contribution of one tissue in the voxel to mask or hide the metabolite contribution from another tissue or abnormality in the voxel. This can be particularly problematic for diagnosing and treating abnormalities such as lesions, tumors, and the like. For example, the metabolite contribution by a cancerous tissue may be masked in conventional MRS by the metabolite contribution of a healthy, non-cancerous tissue also contained in the voxel.
Increasingly, phased array coil architectures are being used for MRS studies. MR imaging with phased array coils is an effective tool for reducing scan time by reducing the number of phase encoding steps during sampling of an MR echo. As is well-known, coil sensitivity information is typically used to supplement phase encode locations in k-space that were not conventionally acquired. In MRS studies, however, phased array acquisitions, albeit providing a scan time reduction, can lead to amplified metabolite signals for a given coil of the array based on that coil's proximity to the voxel in the VOI. In other words, due to the spatially inhomogeneous B1 fields of the individual coils, metabolite signals of tissue closer to the individual coils will be amplified compared to signals of tissue with a large distance from the coils.
It would therefore be desirable to have a system and method capable of single voxel MRS that provides metabolite concentration for a particular tissue of interest contained within the voxel and that is independent of the location of the voxel relative to coils of a phased array configuration.